The eastern North American monarch population is notable for its annual southward late-summer/autumn instinctive migration from the northern and central United States and southern Canada to Florida and Mexico.

[17] In the tenth edition of Systema Naturae, at the bottom of page 467,[18] Linnaeus wrote that the names of the Danai festivi, the division of the genus to which Papilio plexippus belonged, were derived from the sons of Aegyptus.

During their development, both larvae and their milkweed hosts are vulnerable to weather extremes, predators, parasites, and diseases; commonly fewer than 10% of monarch eggs and caterpillars survive.

When searching for nectar, color is the first cue that draws the insect's attention toward a potential food source, and shape is a secondary characteristic that promotes the process.

[70] Overwintering populations of D. p. plexippus are found in Mexico, California, along the Gulf Coast of the United States, year-round in Florida, and in Arizona where the habitat has the specific conditions necessary for survival.

[73] Their wintering habitat typically provides access to streams, plenty of sunlight (enabling body temperatures that allow flight), and appropriate roosting vegetation, and is relatively free of predators.

[74] While breeding, monarch habitats can be found in agricultural fields, pasture land, prairie remnants, urban and suburban residential areas, gardens, trees, and roadsides – anywhere there is access to larval host plants.

[104] The population east of the Rocky Mountains attempts to migrate to the sanctuaries of the Mariposa Monarca Biosphere Reserve in the Mexican state of Michoacán and parts of Florida.

The populations east of the Rocky Mountains, which mostly overwinter in central Mexico, may return the following spring as far north as Texas and Oklahoma before producing offspring to carry the journey northward.

[127] By ingesting a large number of plants in the genus Asclepias, primarily milkweed, monarch caterpillars can sequester cardiac glycosides, or more specifically cardenolides, which are steroids that act in heart-arresting ways similar to digitalis.

[145] A 2012 IMAX film, Flight of the Butterflies, describes the story of the Urquharts, Brugger, and Trail to document the then-unknown monarch migration to Mexican overwintering areas.

In February 2015, the United States Fish and Wildlife Service (USFWS) reported a study that showed that nearly a billion monarchs had vanished from the butterfly's overwintering sites since 1990.

"[163] In 2018, a study correlated monarch butterfly decline to the fact that 95% of corn and soybean crops grown in the United States used genetically modified seeds resistant to the herbicide glyphosate.

Air application of the herbicide meant that the unplowed margins between the field and road that previously supported milkweed and a range of nectar flowers were now greatly diminished.

[165] According to the USFWS, the species faces a host of threats, including the loss and degradation of its breeding, migratory, and overwintering habitats, exposure to insecticides, and the growing impacts of climate change.

[168] A 2016 study attributed the previous decade's 90% decline in overwintering numbers of the eastern monarch population primarily to the loss of breeding habitat and milkweed.

Some conservationists attribute the disappearance of milkweed to agricultural practices in the Midwest, where GM seeds are bred to resist herbicides that farmers use to kill unwanted plants that grow near their rows of food crops.

Mexican environmental authorities continue to monitor illegal logging of the oyamel fir trees; however, organized criminals have repeatedly crushed such efforts in the name of very short-term financial gain.

Parasites include the tachinid flies Sturmia convergens,[185] Compsilura concinnata,[186] Madremyia saundersii,[186] Hyphantrophaga virilis,[186] Nilea erecta,[186] and Lespesia archippivora.

[198] Additionally, milkweed grown at carbon dioxide levels of 760 parts per million was found to produce a different mix of the toxic cardenolides, one of which was less effective against monarch parasites.

[201] On July 20, 2022, the International Union for Conservation of Nature added the migratory monarch butterfly (the subspecies common in North America) to its red list as an endangered species.

[176][177][178] On June 20, 2014, President Barack Obama issued a presidential memorandum entitled "Creating a Federal Strategy to Promote the Health of Honey Bees and Other Pollinators".

Beginning in March 2015, those performance requirements and their updates have included four primary aspects for planting designs that are intended to provide adequate on-site foraging opportunities for targeted pollinators.

The attachment described specific actions that would address the incorporation of pollinator-friendly landscaping design and maintenance into new construction and major renovations, existing sites, contracts, leases and occupancy agreements, and education/outreach programs.

That Plan provided examples of past, ongoing, and possible future collaborations between the federal government and non-federal institutions to support pollinator health under each of the National Strategy's goals.

[223] The USDA's Farm Service Agency helps increase U.S. populations of the monarch butterfly and other pollinators through its Conservation Reserve Program's State Acres for Wildlife Enhancement (SAFE) Initiative.

In addition, national and local initiatives are underway to help establish and maintain pollinator habitats along corridors containing power lines and roadways.

[228] The NCHRP report acknowledged that, among other hazards, roads present a danger of traffic collisions for monarchs, stating that these effects appear to be more concentrated in particular funnel areas during migration.

[229] Nevertheless, the report concluded: In summary, threats along roadway corridors exist for monarchs and other pollinators, but in the context of the amount of habitat needed for recovery of sustainable populations, roadsides are of vital importance.

[231] For example, in the Washington, DC, area and elsewhere in the northeastern and midwestern United States, common milkweed (Asclepias syriaca) is among the most important food plants for monarch caterpillars.